Laser bias and modulation circuit

ABSTRACT

Constant light output is provided by a laser diode over various temperatures or operating points by adjusting the bias current in accordance with a midpoint of a detected peak and a detected valley of an output power level and adjusting the modulation current amplitude in accordance with the difference between the detected peak and the detected valley output power. &lt;IMAGE&gt;   &lt;IMAGE&gt;

TECHNICAL FIELD

This invention relates to the transmission of information on opticalfibers and, more particularly, to a control circuit for controlling alaser for providing optical signals to an optical fiber.

BACKGROUND OF THE INVENTION

Laser diodes are commonly used to provide optical signals to opticalfibers for transmission thereon. Typically, the laser diode will bebiased at some selected bias current level, and the diode will then beintensity-modulated about that bias point at a modulation levelnecessary to achieve a desired light output level. Unfortunately, lasercharacteristics change in two important ways when operated over a widetemperature range. First, the lasing threshold tends to increase withincreasing temperature. This implies that, to maintain a constantaverage optical output power with an increase in temperature, averagedrive current, often called bias current, must be increased. Second, theefficiency of the laser current-to-optical power conversion (known asslope efficiency) decreases with increasing temperature.

One implication of the second effect, i.e., the slope efficiencydecreasing with increasing temperature, is the same as the first: tomaintain at a constant average optical output power with increasingtemperature, bias current must be increased. Another implication of thesecond effect is that, to maintain a constant signal, or modulation,optical output power with increasing temperature, modulation currentmust be increased.

In order to obtain reliable and repeatable results in many fiber optictransmission applications, both average and signal power out of thelaser must be held relatively constant. Many times this problem isskirted through the use of thermo-electric cooling to maintain the laserat a relatively constant temperature. This solution is generally costly,power consumptive, and usually unacceptable for high-volume, low-costapplications. Another possible solution has been to simply monitor thelaser temperature and adjust the bias and signal current levelsaccording to expected performance curves. However, for low-cost lasers,the change in characteristics with temperature is usually not accuratelypredictable from device to device. This mandates that either each laserbe individually characterized over temperature, or that a feedback loopbe established to control the laser in operation. Individualcharacterization, besides being expensive, has the additionaldisadvantage of not accounting for any changes in laser characteristicsthat may occur as the laser ages.

A feedback loop can be established through the laser's own back facetmonitor photodiode, or through the whole link and the receiver at theopposite end. The latter approach has the advantage of being able toaccommodate changes in the cable plant, the receiver, and thelaser-to-fiber coupling. It has the disadvantage of requiring theaddition of control circuitry at the receiver and a link back to thetransmitter. If the feedback link is already present, as it would be fora fiber-to-the-curb application, such as disclosed in U.S. patentapplication Ser. No.07/739,203, entitled "Fiber Optic Link", filed Jul.30, 1991, now abandoned, capacity may be used for feedback information.However, the possibility that feedback information may not arrive backto the laser in a timely fashion, causing instability in the laserperformance, must also be considered.

Localized feedback through the back facet monitor is commonly used toregulate the bias current of the laser. Slope efficiency variations,which are as high as 6 dB, are often ignored. In some cases, through thegeneration and addition of a fixed level `pilot` carrier to the signal,modulation current is also regulated through the back facet monitordiode. However, circuitry must be added to generate the pilot carrierand a multiplexer with the signal. Additionally, link bandwidth is takenup by the pilot.

DISCLOSURE OF INVENTION

The object of the present invention is to regulate a laser.

According to the present invention, constant light output from a laserdiode over various temperatures is achieved by adjusting the biascurrent in accordance with a first set of parameters and adjusting themodulation current amplitude in accordance with a second set ofparameters.

In further accord with the present invention, constant light output overvarious modulation coding schemes from a laser diode is achieved byadjusting the bias current in accordance with a first set of parametersand adjusting the modulation current amplitude in accordance with asecond set of parameters.

In still further accord with the present invention, the first set ofparameters may comprise an average of detected peak and valleyamplitudes of the light output from the laser diode.

In still further accord with the present invention, the second set ofparameters may comprise a difference between the peak and valley of thelight output from the laser diode.

This invention was first conceived as a way to regulate a low-cost,uncooled laser, which was intended to be modulated with a frequencymultiplexed hybrid signal consisting of a relatively low speed (lessthan 50 megabaud) digital signal and a high-speed (up to 1 gigahertz)analog composite video signal. The application for the invention was ina fiber-to-the-curb system, such as disclosed in the above-referencedcopending U.S. patent application Ser. No. 07/739,203, now abandonedwhere wide temperature ranges must be handled by low-cost designs, andwhere the transmitted signal is a hybrid analog video/digital voicesignal.

Thus, the present invention provides a way to use the already-presentdigital signal in an all-digital or all-hybrid digital/analogapplication as a virtual pilot. We also teach a technique for using thedigital signal such that most non-balanced coding schemes anddata-dependent DC-based line wandering in the digital signal will notincorrectly affect the laser bias and modulation currents.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows laser bias and modulation amplitude varied in order toachieve constant output power, according to the present invention.

FIG. 2 shows the relation between FIGS. 2(a) and 2(b).

FIG. 2(a) and 2(b) together show a block diagram of a circuit, accordingto the present invention, for automatic bias and modulation control fora hybrid digital/analog application according to the present invention.

FIG. 3 is an illustration of an embodiment of the laser driver of FIGS.2(a) and 2(b) for handling the digital data signal.

FIG. 4 is an illustration of an embodiment of the laser driver of FIG. 2for handling the high-speed signals.

FIG. 5 shows the relation between FIGS. 5(a) and 5(b).

FIGS. 5(a) and 5(b) together are a detailed illustration of one way tocarry out the bias and automatic gain control circuitry of FIGS. 2(a)and 2(b).

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows that a constant output power may be achieved for varyingefficiencies or operating points of a laser diode modulated by amodulating signal by controlling the bias point and the amplitude of themodulation. In the case of varying efficiencies as caused, for example,by varying temperatures, the present invention automatically compensatesby varying the operating point and the amplitude of modulation. This isshown for three different temperatures, resulting in three differentoperating points and amplitudes of the modulating signal. In the case ofvarying operation points which could be caused, for example, by adigital modulating signal without a 50 percent duty cycle, though theoperating point would have a tendency to deviate to some extent (as inthe prior art) to the left or right of one of the operating points(I₁,I₂, I₃) as shown in FIG. 1, according to the present invention, theoperating point is made to the same.

FIGS. 2(a) and 2(b) show an embodiment of the present invention whichincludes a feedback circuit 10 used to regulate a low-cost, uncooledlaser 12, which is being modulated by a frequency-division multiplexedsignal 14 which is shown before being multiplexed within a driver 20 astwo separate signals on a pair of lines 16, 18. The components 16, 18 ofthe signal 14 may comprise a relatively low-speed digital data signal(for example, at less than 50 megabaud) and one or more high-speed RFsignals such as analog television channels (spaced along the frequencyspectrum up to 1 gigahertz, for example). The laser driver circuit 20 isresponsive to the signals on the lines 16, 18, multiplexes the signals16, 18 into the frequency-division multiplexed signal 14 and providesdrive current for the laser 12, which in turn provides light output 22to an optical fiber (not shown) for transmission thereon. The laserdriver modulates the laser around a bias current level and transmitslight 22 into the optical fiber through its front facet and alsotransmits light 24 into a monitor photodiode 26 through its back facet.The frequency response of the monitor diode is generally slow enough toblock the analog video signals, but fast enough to pass the digitalsignal. However, if the analog video signals are not significantlyattenuated at the monitor diode, then a lowpass filter 28 may beinserted in the signal path to filter them out. In any event, either thelaser back facet monitor diode 26 or the lowpass filter, or both, willprovide a filtered low-frequency signal on a line 30 to acurrent-to-voltage level converter 32, which may be nothing more than aresistor network to a reference to provide a voltage on the same line 30to a voltage peak-to-peak detector 36a, 36b.

The peak and valley detection circuits 36a, 36b, respectively, where thehighest and lowest absolute voltage levels found in the convertedsignals are held and provided on lines 38a, 38b, respectively. Theresponse and hold times of the peak and valley detectors are setdepending upon the symbol rate of the digital data and the longestduration between transitions. It should be realized that truenon-return-to-zero (NRZ) encoding, where the time between symboltransitions can be extremely long, cannot be used with the embodimentdisclosed herein.

After peak and valley detection, the control voltages on the lines 38a,38b are provided to an averager 40 and a difference detector 42. In theaverager, the peak and valley voltages 38a, 38b are averaged to find themidpoint, and a midpoint signal having a magnitude indicative thereof isprovided on a line 44 to a laser bias control 46. Because peak andvalley voltages are used to determine the midpoint, instead ofperforming average level detection immediately after the back facetmonitor, data and coding scheme dependent wandering of the average levelis avoided. If the coding scheme utilized does not have a true 50percent effective duty cycle where one is guaranteed a transition indata and between bits, any variations that would otherwise be caused inlaser bias control due to variations away from 50 percent duty cycle areavoided.

The midpoint voltage on the line 44 is provided to the laser biascontrol block 46, where it is compared to a threshold signal on a line48 which may be adjustable, as indicated by a threshold adjust block 50.The threshold is chosen to place the laser at a selected average opticaloutput power level, such as the P₀ level shown in FIG. 1. If themidpoint voltage is below the threshold, then the laser bias controlblock provides a signal on a line 52 to increase the bias current in thelaser driver block 20 until the midpoint voltage equals the threshold.An opposite effect occurs if the midpoint voltage is above threshold.

In the difference detector 42, the difference between the peak andvalley voltages on the lines 38a, 38b, respectively, is determined. Adifference voltage signal is provided on a line 54 to a laser modulationcontrol block 56, where it is compared to a threshold signal on a line58 provided by a threshold adjust block 60 which may be capable ofadjusting the level of the threshold reference signal on the line 58.The threshold is set according to the desired modulation index or ratioof signal-power-to-average-power out of the laser of the digital signal.If the difference voltage is below threshold, then the laser modulationcontrol block provides a signal on a line 62 to increase the gain of thesignal in the laser driver 20. An opposite effect occurs if thedifference voltage is above threshold. In the driver, both the digitaland analog signals are adjusted proportionately, so that the relativemodulation indices set up for the two signal types is maintained.

Turning now to FIG. 3, a detailed illustration is shown of one way toconstruct a portion of the laser driver 20 of FIGS. 2(a) and 2(b) forhandling the relatively low-speed digital data input signal on the line16. That signal is first terminated by a resistor 70 and capacitor 72 inparallel, which helps with proper switching of a transistor 74 toprevent pulsewidth distortion. The transistor 74 is sourced by a VCCvoltage at a node 76 at +5V. The collector voltage is filtered by acapacitor pair 78 in parallel, and a series of resistors 79 is employedfor the purpose of level conversion before being buffered by a bufferstage 80, which also serves to convert the driving signal to a drivingsignal having negative excursions. This was done for the particularembodiment shown because of the particular laser selected, which needsto be driven negatively. This, of course, need not be the case.

An emitter follower section 82 follows the buffer stage 80 and furtherlevel converts the input to get closer to a negative 5V suited to theparticular laser diode 12 we selected. A buffer section 84 buffers theoutput of the emitter follower section 82 and is itself followed by alowpass filter section 86 which passes signals, for example in the rangeof 0-20 MHz. This is not the same as the lowpass filter 28 shown inFIGS. 2(a) and 2(b) but is merely provided to eliminate higher frequencycomponents from the digital signal which would otherwise causedistortions in the analog video signal, to be multiplexed with thedigital at node 102.

The lowpass filter 86 is followed by a further conversion section 88which places the most negative excursion of the filtered signal almostdown to -5V so that a minimal DC current will be provided by thissection.

For given signal excursion or swing at the base of transistor 90, theresistor 91 sets the modulation current for the digital signal in thelaser 12.

The light output 24 from the laser diode 12 is detected by the backfacet 26 thereof, and the detected signal is provided on the line 30, asdescribed above in connection with FIGS. 2(a) and 2(b). Although alowpass filter 28 is shown in FIGS. 2(a) and 2(b), for the particularcase illustrated, it was found not necessary to include such a lowpassfilter, since the capacitance of the diode itself plus the inputresistance of the transistor 154 circuit was sufficiently great toprovide an effective lowpass filter, in this particular case.

In connection with the description of FIG. 4 below, it will be seen thatthe output thereof is provided on a line 100 to a summing node 102 shownin FIG. 3, where the output of the video circuit of FIG. 4 is summedwith the current contributed by the digital portion illustrated in FIG.3. This effectively forms a frequency-division multiplexing node formultiplexing the low-frequency digital signals with the high-frequencyvideo signals. The multiplexed signal thus modulates the diode 12.

Turning now to FIG. 4, an illustration is shown of video circuitry for alaser driver 20 embodiment, according to present invention. Thehigh-speed RF signals 18 of FIGS. 2(a) and 2(b) are shown at the upperleft of FIG. 4 as an input signal to a voltage controlled attenuator 104to keep the video level at a constant level as controlled by a signal ona line 106, which is in turn controlled by the feedback signal on theline 62 shown in both. FIGS. 2(a) and 2(b) and FIG. 4 (from the lasermodulation control 56 to be disclosed in more detail below in connectionwith FIGS. 5(a) and 5(b).

An output signal on a line 106 from the voltage controlled attenuator104 is filtered by a highpass filter 108 for getting rid of any lowfrequency components that might otherwise corrupt the digital signals tobe joined with the video signals on the line 100, as shown summed at thejunction 102 in FIG. 3.

The signal on the line 106 is also provided to a series of resistors109, 110 and a capacitor 112, which together serve the purpose ofpresenting an impedance which prevents any influence of the controlcircuitry on the video signal path 106. In other words, resistors 109,110 and the capacitor 112 have values selected such that the impedancepresented to the main line 106 is greater than ten times thecharacteristic impedance of the main line 106. A diode 114 (configuredas a back diode to minimize temperature drift effects) and capacitor 116serve the purpose of doing average power detection. The capacitor 116also acts in concert with a resistor 118 as a pre-integrator to preventinstantaneous high peaks from being presented to an integrator 120 whichintegrates a signal on a line 122, to provide an integrated outputsignal on a line 124. The integrator is with reference to a referencesignal on a line 126, whereby the integrator 120 tries to zero thedifference between the signal on the line 122 and the signal on the line126. After the integrator 120, stages 128, 130 perform both gain andlevel shifting by moving the DC swing of a signal on line 132 into arange that the voltage-controlled attenuator 104 needs as a controlvoltage, i.e., between 0 and -5V.

The differential amplifier stage 130 drives the difference between theinput signal on the line 132 and an input signal on a line 134 to zeroThe signal on the line 134 is provided by a signal conditioning stage136 response to the feedback signal on the line 62 from FIGS. 5(a) and5(b).

FIGS. 5(a) and 5(b) shows the laser bias and AGC control part of FIGS.2(a) and 2(b). A voltage regulator 150 is responsive to the -5V on line92 for providing a regulated a -2.5V on a line 152. At equilibrium, thevoltage on the line 34 sits at -1.25V, and the amount of swing isdetermined by the way the modulation index is set up.

A transistor 154 is responsive at its collector to the +5V on the line76 and provides a current through a pair of resistors 156, 158, whichhave a connecting node 160 which, at equilibrium, sits at -2.5V. Thepeak detector 36a and valley detector 36b of FIGS. 2(a) and 2(b) areshown in one embodiment in FIG. 5(a) and 5(b). A buffer operationalamplifier 162 provides the captured peak signal on the line 38a, whichis held by a circuit 164 having a resistor 166 and a capacitor 168 withvalues chosen so as to have a time constant longer than the longestexpected time between transitions of the digital input signal, which maybe unbalanced in the sense of not having a 50 percent duty cycle. Thus,for a case where the longest time between transitions is on the order of180 nanoseconds, the resistive value may be chosen to be 162 Kohm, whilethe capacitor may be about 33 picofarad. Similar values can be chosenfor a resistor 170 and a capacitor 172 in a circuit 174 having a purposeof holding the peak or valley of the signal on the line 160. Anoperational amplifier 176 provides the valley signal on the line 38b tothe averager 40, which is responsive to both the signal on the line 38band to the signal on the line 38a for finding the midpoint. Adifferential amplifier 180 compares the reference signal on the line 48to the average signal on the line 44, as previously discussed inconnection with FIGS. 2(a) and 2(b). The output of the differentialamplifier 180 is the signal on the line 52 shown in FIGS. 2(a) and 2(b)as a feedback signal for the laser driver. This signal is used to drivea transistor 181. For a given signal voltage at the base of transistor181, a DC or bias current is set by resistor 181a and is then summedthrough line 182 at node 102 with the video and digital modulationcurrents passing through the laser diode 12.

The signal on the line 38a is a signal representing the latest peakvalue of the input signal. It is provided not only to the voltagefollower 40 of the average detector, but also the difference detector 42which is responsive also to the signal on the line 38b for providing thedifference signal on the line 54 to a comparator 190, which compares thedifference signal on the line 54 to the reference signal on the line 58and provides the feedback signal on the line 62 to the video portion ofthe laser driver, as shown in FIG. 4. The signal on the line 62 is alsoused at the gate of an FET 192 used as a voltage-controlled resistor inthe digital portion of the laser driver circuit of FIG. 3. The signalchanges the resistance of the FET in accordance with changes in thedifference in the peak and valley, therefore changing the voltagedivision ration of the digital signal after transistor 80 in FIG. 3.

Although the invention has been shown and described with respect to abest mode embodiment thereof, it should be understood by those skilledin the art that the foregoing and various other changes, omissions andadditional in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method for maintaining constant light outputfor varying efficiencies and operating points of a laser diode modulatedby a modulating signal, comprising the steps of:sensing the light outputand providing a sensed signal having a magnitude indicative thereof;providing, in response to the sensed signal, a bias current feedbackcontrol signal for controlling a bias current provided to the laserdiode; and providing, in response to the sensed signal, a modulationcurrent amplitude feedback control signal for controlling the modulationcurrent amplitude used to modulate the bias current.
 2. The method ofclaim 1, wherein the varying efficiencies are due to varyingtemperatures.
 3. The method of claim 1, wherein the varying operatingpoints are due to variations in duty cycle of the modulating signal atleast partly in digital form.
 4. The method of claim 1, wherein the biascurrent feedback control signal is provided according to a midpoint ofpeaks of the sensed signal.
 5. The method of claim 1, wherein themodulation current amplitude feedback control signal is providedaccording to a difference in magnitudes between peaks of the sensedsignal.
 6. The method of claim 1, wherein the modulating signalcomprises a relatively low-speed digital data signal frequency-divisionmultiplexed with one or more relatively high-speed signals.
 7. Themethod of claim 6, wherein the high-speed signals are analog videosignals.
 8. Apparatus for maintaining constant light output for varyingefficiencies and operating points of a laser diode modulated by amodulating signal, comprising:means responsive to the light output forproviding a sensed signal having a magnitude indicative thereof; meansfor providing, in response to the sensed signal, a bias current feedbackcontrol signal for controlling a bias current provided to the laserdiode; means for providing, in response to the sensed signal, amodulation current amplitude feedback control signal for controlling themodulation current amplitude used to modulate the bias current; andlaser driver means, responsive to the bias current feedback controlsignal and the modulation current amplitude feedback control signal andto the modulating signal for providing the constant light output.
 9. Theapparatus of claim 8, wherein the varying efficiencies are due tovarying temperatures.
 10. The apparatus of claim 8, wherein the varyingoperating points are due to variations in duty cycle of the modulatingsignal at least partly in digital form.
 11. The apparatus of claim 8,wherein the bias current feedback control signal is provided accordingto a midpoint of peaks of the sensed signal.
 12. The apparatus of claim8, wherein the modulation current amplitude feedback control signal isprovided according to a difference in magnitude between peaks of thesensed signal.
 13. The apparatus of claim 8, wherein the modulatingsignal comprises a relatively low-speed digital data signalfrequency-division multiplexed with one or more relatively high-speedsignals.
 14. The apparatus of claim 13, wherein the high-speed signalsare analog video signals.
 15. Apparatus, comprising:a laser diode driver(20), responsive to a low-speed digital data signal (16), a high-speedRF signal (18), a bias feedback signal (52) and a modulation feedbacksignal (62), for providing a driving signal (102); a laser diode (12),responsive to the driving signal (102), for providing a constant lightoutput signal (22) and a monitoring light output signal (24); a backfacet monitor diode (26), responsive to the monitoring light outputsignal (24), for providing a sensed signal (30); peak detection means(36a), responsive to the sensed signal (30), for providing a peak signal(38a) having a magnitude indicative of a positive peak amplitude of thesensed signal; valley detection means, responsive to the sensed signal(30), for providing a valley signal (38b) having a magnitude indicativeof a negative peak amplitude of the sensed signal; averager means (40),responsive to the peak and valley signals (38a, 38b), for providing amidpoint signal (44) having a magnitude indicative of a midpoint betweenthe positive and negative peak signals; difference detector means (42),responsive to the peak and valley signals (38a, 38b), for providing adifference signal (54) having a magnitude indicative of a differencebetween the positive and negative peak signals; laser bias control means(46), responsive to the midpoint signal (44) and to a bias referencesignal (48), for providing the bias feedback signal (52); and lasermodulation control (56), responsive to the difference signal (54) and toa modulation reference signal (58), for providing the modulationfeedback signal (62).
 16. The apparatus of claim 15, further comprisingcurrent-to-voltage level conversion means (32), responsive to the sensedsignal (30), for providing the sensed signal (30) in converted form. 17.The apparatus of claim 15, further comprising lowpass filter means,responsive to the sensed signal (30) for providing a low-pass filteredsensed signal (30).